[Technical Field]
[0001] The present invention relates to a steel sheet for hot pressing use used in manufacturing
structural components of an automobile and suitable for hot press forming, a press-formed
product obtained from such a steel sheet for hot pressing use, and a method for manufacturing
the press-formed product, and relates more specifically to a steel sheet for hot pressing
use that is useful in being applied to a hot press forming method securing a predetermined
strength by being subjected to heat treatment simultaneously with impartation of the
shape in forming a pre-heated steel sheet (blank) into a predetermined shape, a press-formed
product, and a useful method for manufacturing such a press-formed product.
[Background Art]
[0002] As one of the fuel economy improvement measures of an automobile triggered by global
environment problems, weight reduction of the vehicle body is advancing, and it is
necessary to high-strengthen a steel sheet used for an automobile as much as possible.
On the other hand, when a steel sheet is high-strengthened, shape accuracy in press
forming comes to deteriorate.
[0003] On this account, a hot press forming method has been employed for manufacturing components
in which a steel sheet is heated to a predetermined temperature (for example, a temperature
at which a state of an austenitic phase is achieved), the strength is lowered, the
steel sheet is thereafter formed using a tool of a temperature (room temperature for
example) lower than the steel sheet, thereby impartation of a shape and rapid heat
treatment (quenching) utilizing the temperature difference of the both are executed
simultaneously, and the strength after forming is secured. Also, such a hot-press
forming method is referred to by various names such as a hot forming method, hot stamping
method, hot stamp method, die quench method, and the like in addition to the hot press
method.
[0004] Fig. 1 is a schematic explanatory drawing showing a tool configuration for executing
hot press forming described above, 1 in the drawing is a punch, 2 is a die, 3 is a
blank holder, 4 is a steel sheet (blank), BHF is a blank holding force, rp is punch
shoulder radius, rd is die shoulder radius, and CL is punch/die clearance respectively.
Also, out of these components, in the punch 1 and the die 2, passages 1a, 2a through
which a cooling medium (water for example) can pass are formed inside of each, and
it is configured that these members are cooled by making the cooling medium pass through
these passages.
[0005] In hot press forming (hot deep drawing for example) using such a tool, forming is
started in a state the steel sheet (blank) 4 is heated to a two-phase zone temperature
(between Ac
1 transformation point and Ac
3 transformation point) or a single-phase zone temperature of Ac
3 transformation point or above and is softened. That is, in a state the steel sheet
4 in a high temperature state is sandwiched between the die 2 and the blank holder
3, the steel sheet 4 is pressed in to the inside of a hole of the die 2 by the punch
1, and is formed into a shape corresponding to the shape of the outer shape of the
punch 1 while reducing the outside diameter of the steel sheet 4. Also, by cooling
the punch 1 and the die 2 in parallel with forming, heat removal from the steel sheet
4 to the tools (the punch 1 and the die 2) is executed, holding and cooling are further
executed at a forming bottom dead point (the temporal point the tip of the punch is
positioned at the deepest point: the state shown in Fig. 1), and thereby quenching
of the raw material is executed. By executing such a forming method, a formed product
of 1,500 MPa class with excellent dimensional accuracy can be obtained, the forming
load can be reduced compared with a case a component of a same strength class is cold-formed,
and therefore less capacity of the press machine is needed.
[0006] As a steel sheet for hot pressing use widely used at present, one using 22Mn-B5 steel
as a raw material is known. The steel sheet has the tensile strength of approximately
1,500 MPa and the elongation of approximately 6-8%, and is applied to a shock resistant
member (a member not causing deformation as much as possible and not causing breakage
in collision). However, application to a component requiring deformation such as an
energy absorption member is difficult because elongation (ductility) is low.
[0007] As a steel sheet for hot pressing use exerting excellent elongation, technologies
such as the patent literatures 1-4 for example have also been proposed. According
to these technologies, the basic strength class of each steel sheet is adjusted by
setting the carbon content in the steel sheet to various ranges, and elongation is
improved by introducing ferrite with high deformability and reducing the average grain
size of ferrite and martensite. Although these technologies are effective in improving
elongation, they are still insufficient from the viewpoint of improving elongation
matching the strength of the steel sheet. For example, those having 1,470 MPa or more
of the tensile strength TS have the elongation EL of approximately 10.2% at the maximum,
and further improvement is required.
[0008] On the other hand, even the formed product having a lower strength class compared
to the hot stamp formed product, 980 MPa class or 1,180 MPa class of the tensile strength
TS for example, having been studied until now has a problem in forming accuracy of
cold press forming, and there are needs for low-strength hot press forming as an improvement
measure therefor. At that time, it is necessary to largely improve energy absorption
properties in the formed product.
[0009] Particularly, in recent years, development of the technology for differentiating
the strength within a single component is proceeding. As such a technology, a technology
has been proposed in which the portion that must be prevented from deforming has high
strength (high strength side: shock resistant portion side), and the portion that
needs energy absorption has low strength and high ductility (low strength side: energy
absorption portion side). For example, in a passenger car of the middle class or above,
there is a case that portions having both functions of shock resistant property and
energy absorption property are provided within a component of a B-pillar and rear
side member considering compatibility in a side collision and a rear collision (a
function for protecting the counterpart side also when a small-sized car collides
with). For the purpose of manufacturing such members, (a) a method of joining a steel
sheet becoming of low strength even in being heated to a same temperature and tool-quenched
to a normal steel sheet for hot pressing use (tailored weld blank: TWB), (b) a method
for differentiating the strength for each region of a steel sheet by differentiating
the cooling rate in the tool, (c) a method for differentiating the strength by differentiating
the heating temperature for each region of a steel sheet, and the like have been proposed.
[0010] Although the tensile strength: 1,500 MPa class is achieved on the high strength side
(shock resistant portion side) according to these technologies, the maximum tensile
strength is 700 MPa and the elongation EL is approximately 17% on the low strength
side (energy absorption portion side), and achievement of higher strength and higher
ductility are required in order to further improve the energy absorption properties.
[Citation List]
[Patent Literature]
[Summary of Invention]
[Technical Problems]
[0012] The present invention has been developed in view of such circumstances as described
above, and its object is to provide a steel sheet for hot pressing use capable of
obtaining a hot press-formed product that can achieve the balance of high strength
and elongation with a high level when uniform property is required within a formed
product and is useful in obtaining a press-formed product that can achieve the balance
of high strength and elongation with a high level according to each region when regions
corresponding to a shock resistant portion and an energy absorption portion are required
within a single formed product, a press-formed product exerting the properties described
above, and a useful method for manufacturing such a hot press-formed product.
[Solution to Problems]
[0013] The steel sheet for hot pressing use of the present invention which could achieve
the object described above contains:
C: 0.15-0.5% (means mass%, hereinafter the same with respect to the chemical component
composition);
Si: 0.2-3%;
Mn: 0.5-3%;
P: 0.05% or less (exclusive of 0%);
S: 0.05% or less (exclusive of 0%);
Al: 0.01-1%;
B: 0.0002-0.01%;
Ti: 3.4[N]+0.002% or more and 3.4[N]+0.1% or less ([N] expresses N content (mass%)),
and
N: 0.001-0.01% respectively, with the remainder consisting of iron and inevitable
impurities, in which
some of Ti-containing precipitates contained in the steel sheet, each of which having
an equivalent circle diameter of 30 nm or less, have an average equivalent circle
diameter of 3 nm or less, the precipitated Ti amount and the total Ti amount in the
steel fulfill the relationship represented by formula (1) shown below, and the sum
total of the fraction of bainite and the fraction of martensite in the metal microstructure
is 80 area% or more. Also, "equivalent circle diameter" is the diameter of an imaginary
circle having an area same to the size (area) of Ti containing precipitates (TiC for
example) ("the average equivalent circle diameter" is the average value thereof).
(In the formula (1), [N] represents the content (mass%) of N in the steel.)
[0014] In the steel sheet for hot pressing use of the present invention, according to the
necessity, it is also useful to contain, as other elements, (a) at least one element
selected from the group consisting of V, Nb and Zr by 0.1% or less (exclusive of 0%)
in total, (b) at least one element selected from the group consisting of Cu, Ni, Cr
and Mo by 1% or less (exclusive of 0%) in total, (c) at least one element selected
from the group consisting of Mg, Ca and REM by 0.01% or less (exclusive of 0%) in
total, and the like, and the properties of the press-formed product is improved further
according to the kind of the elements contained.
[0015] The method for manufacturing a press-formed product of the present invention which
could achieve the object described above includes the steps of using such a steel
sheet for hot pressing use of the present invention as described above, heating the
steel sheet to a temperature of Ac
1 transformation point+20°C or above and Ac
3 transformation point-20°C or below, thereafter starting press forming, and executing
cooling to a temperature or below, the temperature being lower than the bainite transformation
starting temperature Bs by 100°C, while securing the average cooling rate of 20°C/s
or more within a tool during forming and after completion of forming.
[0016] In the press-formed product obtained by the method for manufacturing, the metal microstructure
includes retained austenite: 3-20 area%, annealed martensite and/or annealed bainite:
30-87 area%, and martensite as quenched: 10-67 area%, the amount of carbon in the
retained austenite is 0.60% or more, and the balance of high strength and elongation
can be achieved with a high level and as a uniform property within the formed product.
Also, the area ratio of annealed martensite and/or annealed bainite means the total
area ratio of both microstructures when both microstructures are included, and means,
when either one microstructure is included, the area ratio of the microstructure.
[0017] Also, another method for manufacturing a press-formed product of the present invention
which could achieve the object described above includes the steps of using such a
steel sheet for hot pressing use of the present invention as described above, dividing
a heating region of the steel sheet into two regions, heating one region thereof to
a temperature of Ac
3 transformation point or above and 950°C or below, heating the other region to a temperature
of Ac
1 transformation point+20°C or above and Ac
3 transformation point-20°C or below, thereafter starting press forming, and executing
cooling to a temperature of martensite transformation starting temperature Ms or below
while securing the average cooling rate of 20°C/s or more within a tool during forming
and after completion of forming.
[0018] In the press-formed product obtained by the method for manufacturing, a first region
whose metal microstructure includes retained austenite: 3-20 area% and martensite:
80 area% or more and a second region whose metal microstructure includes retained
austenite: 3-20 area%, annealed martensite and/or annealed bainite: 30-87 area%, and
martensite as quenched: 10-67 area% with the amount of carbon in the retained austenite
being 0.60% or more are included, the balance of high strength and elongation can
be achieved with a high level according to each region, and regions corresponding
to a shock resistant portion and an energy absorption portion are present within a
single formed product.
[Advantageous Effects of Invention]
[0019] According to the present invention, because a steel sheet is used in which the chemical
component composition is strictly stipulated, the size of Ti-containing precipitates
is controlled, the precipitation rate is controlled for Ti that does not form TiN,
and the ratio of tempered hard phase (martensitic phase, bainitic phase and the like),
hard phase (as-quenched martensite phase) and retained austenite phase is adjusted
with respect to the metal microstructure, by hot-pressing the steel sheet under a
predetermined condition, high strength-elongation balance of the press-formed product
can be made a high level. Also, when hot-pressing is executed under different conditions
in plural regions, the shock resistant portion and the energy absorption portion can
be formed within a single formed product, the balance of high strength and elongation
can be achieved with a high level for each portion.
[Brief Description of Drawings]
[0020] [Fig. 1] Fig. 1 is a schematic explanatory drawing showing a tool configuration for
executing hot press forming.
[Description of Embodiments]
[0021] The present inventors carried out studies from various aspects in order to achieve
such a steel sheet for hot pressing use that can obtain a press-formed product exhibiting
excellent ductility (elongation) also while securing high strength after press-forming
in manufacturing the press-formed product by heating a steel sheet to a predetermined
temperature and thereafter executing hot press forming.
[0022] As a result of the studies, it was found out that, when the chemical component composition
of the steel sheet for hot pressing use was strictly stipulated, the size of Ti-containing
precipitates and precipitated Ti amount were controlled and the metal microstructure
was made an appropriate one, by hot press forming of the steel sheet under a predetermined
condition, a press-formed product in which retained austenite of a predetermined amount
was secured after press forming and intrinsic ductility (residual ductility) was enhanced
could be obtained, and the present invention was completed.
[0023] In the steel sheet for hot pressing use of the present invention, it is necessary
to strictly stipulate the chemical component composition, and the reasons for limiting
the range of each chemical component are as follows.
[C: 0.15-0.5%]
[0024] C is an important element in achieving the balance of high strength and elongation
of a case uniform properties are required within a formed product with a high level
or in securing retained austenite particularly in the low strength/high ductility
portion of a case the regions corresponding to a shock resistant portion and an energy
absorption portion are required within a single formed product. Also, by concentration
of C to austenite in heating of hot press forming, retained austenite can be formed
after quenching. Also, C contributes to increase of the amount of martensite, and
increases the strength. In order to exert such effects, C content should be 0.15%
or more.
[0025] However, when C content becomes excessive and exceeds 0.5%, two phase zone heating
range becomes narrow, and the balance of high strength and elongation of a case uniform
properties are required within a formed product is not achieved with a high level,
or it becomes hard to adjust the metal microstructure to that targeted particularly
in the low strength/high ductility portion (a microstructure in which a predetermined
amount of annealed martensite and/or annealed bainite is secured) of a case the regions
corresponding to a shock resistant portion and an energy absorption portion are required
within a single formed product. Preferable lower limit of C content is 0.17% or more
(more preferably 0.20% or more), and more preferable upper limit is 0.45% or less
(further more preferably 0.40% or less).
[Si: 0.2-3%]
[0026] Si exerts an effect of forming retained austenite by suppressing that martensite
is tempered during cooling of tool-quenching and cementite is formed, or that untransformed
austenite is disintegrated. In order to exert such an effect, Si content should be
0.2% or more. Also, when Si content becomes excessive and exceeds 3%, ferrite is liable
to be formed, formation of single-phase microstructure becomes hard in heating, and
required fractions of bainite and martensite cannot be secured in a steel sheet for
hot pressing use. Preferable lower limit of Si content is 0.5% or more (more preferably
1.0% or more), and preferable upper limit is 2.5% or less (more preferably 2.0% or
less).
[Mn: 0.5-3%]
[0027] Mn is an element effective in enhancing quenchability and suppressing formation of
a microstructure (ferrite, pearlite, bainite and the like) other than martensite and
retained austenite during cooling of tool-quenching. Also, Mn is an element stabilizing
austenite, and is an element contributing to increase of retained austenite amount.
In order to exert such effects, Mn should be contained by 0.5% or more. Although Mn
content is preferable to be as much as possible when only properties are considered,
because the cost of adding alloy increases, Mn content is made 3% or less. Preferable
lower limit of Mn content is 0.7% or more (more preferably 1.0% or more), and preferable
upper limit is 2.5% or less (more preferably 2.0% or less).
[P: 0.05% or less (exclusive of 0%)]
[0028] Although P is an element inevitably included in steel, because P deteriorates ductility,
P is preferable to be reduced as much as possible. However, because extreme reduction
causes increase of the steel making cost and to make it 0% is difficult in manufacturing,
P content is made 0.05% or less (exclusive of 0%). Preferable upper limit of P content
is 0.045% or less (more preferably 0.040% or less).
[S: 0.05% or less (exclusive of 0%)]
[0029] Similar to P, S is also an element inevitably included in steel, S deteriorates ductility,
and therefore S is preferable to be reduced as much as possible. However, because
extreme reduction causes increase of the steel making cost and to make it 0% is difficult
in manufacturing, S content is made 0.05% or less (exclusive of 0%). Preferable upper
limit of S content is 0.045% or less (more preferably 0.040% or less).
[Al: 0.01-1%]
[0030] Al is useful as a deoxidizing element, fixes solid-solution N present in steel as
AlN, and is useful in improving ductility. In order to effectively exert such an effect,
Al content should be 0.01% or more. However, when Al content becomes excessive and
exceeds 1%, Al
2O
3 is formed excessively, and ductility is deteriorated. Also, preferable lower limit
of Al content is 0.02% or more (more preferably 0.03% or more), and preferable upper
limit is 0.8% or less (more preferably 0.6% or less).
[B: 0.0002-0.01%]
[0031] B is an element contributing to prevention of formation of ferrite, pearlite and
bainite during cooling after heating to a two-phase zone temperature of (Ac
1 transformation point - Ac
3 transformation point) because B has an action of suppressing ferrite transformation,
pearlite transformation and bainite transformation on the high strength portion side,
and to secure retained austenite. In order to exert such effects, B should be contained
by 0.0002% or more, however, even when B is contained excessively exceeding 0.01%,
the effects saturate. Preferable lower limit of B content is 0.0003% or more (more
preferably 0.0005% or more), and preferable upper limit is 0.008% or less (more preferably
0.005% or less).
[Ti: 3.4[N]+0.01% or more and 3.4[N]+0.1% or less: [N] expresses N content (mass%)]
[0032] Ti develops improvement effect of quenchability by fixing N and holding B in a solid
solution state. In order to exert such an effect, it is important to contain Ti more
than the stoichiometric ratio of Ti and N (3.4 times of N content) by 0.01% or more.
However, when Ti content becomes excessive to be more than 3.4[N]+0.1%, Ti-containing
precipitates formed are finely dispersed and impede the growth of martensite during
cooling after heating to the two phase zone temperature, a lath (lath-like martensite)
with a small aspect ratio is formed, discharging of carbon (C) to retained austenite
between the laths becomes slow, and the carbon amount in the retained austenite reduces.
Preferable lower limit of Ti content is 3.4[N]+0.02% or more (more preferably 3.4[N]+0.05%
or more), and preferable upper limit is 3.4[N]+0.09% or less (more preferably 3.4[N]+0.08%
or less).
[N: 0.001-0.01%]
[0033] N is an element inevitably mixed in and is preferable to be reduced, however, because
there is a limit in reducing N in an actual process, 0.001% is made the lower limit.
Also, when N content becomes excessive, the ductility deteriorates because of time
aging, N precipitates as BN, the quenchability improvement effect by solid-dissolved
B is deteriorated, and therefore the upper limit is made 0.01%. Preferable upper limit
of N content is 0.008% or less (more preferably 0.006% or less).
[0034] The basic chemical composition in the steel sheet for hot pressing use of the present
invention is as described above, and the remainder is iron and inevitable impurities
other than P, S (O, H and the like for example). Further, in the steel sheet for hot
pressing use of the present invention, according to the necessity, it is also useful
to further contain (a) at least one element selected from the group consisting of
V, Nb and Zr by 0.1% or less (exclusive of 0%) in total, (b) at least one element
selected from the group consisting of Cu, Ni, Cr and Mo by 1% or less (exclusive of
0%) in total, (c) at least one element selected from the group consisting of Mg, Ca
and REM (rare earth elements) by 0.01% or less (exclusive of 0%) in total, and the
like, and the properties of the steel sheet for hot pressing use are improved further
according to the kind of the element contained. Preferable range when these elements
are contained and reasons for limiting the range are as follows.
[At least one element selected from the group consisting of V, Nb and Zr by 0.1% or
less (exclusive of 0%) in total]
[0035] V, Nb and Zr have effects of forming fine carbide and miniaturizing the microstructure
by a pinning effect. In order to exert such effects, it is preferable to contain them
by 0.001% or more in total. However, when the content of these elements becomes excessive,
coarse carbide is formed and becomes a start point of breakage, and ductility is deteriorated
adversely. Therefore, it is preferable to contain these elements by 0.1% or less in
total. More preferable lower limit of the content of these elements in total is 0.005%
or more (further more preferably 0.008% or more), and more preferable upper limit
in total is 0.08% or less (further more preferably 0.06% or less).
[At least one element selected from the group consisting of Cu, Ni, Cr and Mo: 1%
or less (exclusive of 0%) in total]
[0036] Cu, Ni, Cr and Mo suppress ferrite transformation, pearlite transformation and bainite
transformation, therefore prevent formation of ferrite, pearlite and bainite during
cooling after heating, and act effectively in securing retained austenite. In order
to exert such effects, it is preferable to contain them by 0.01% or more in total.
Although the content is preferable to be as much as possible when only the properties
are considered, because the cost for adding alloys increases, 1% or less in total
is preferable. Also, because there is an action of largely increasing the strength
of austenite, the load of hot rolling increases, manufacturing of the steel sheet
becomes difficult, and therefore 1% or less is also preferable from the viewpoint
of manufacturability. More preferable lower limit of these elements in total is 0.05%
or more (further more preferably 0.06% or more), and more preferable upper limit in
total is 0.5% or less (further more preferably 0.3% or less).
[At least one element selected from the group consisting of Mg, Ca and REM by 0.01%
or less (exclusive of 0%) in total]
[0037] Because these elements miniaturize inclusions, they act effectively in improving
ductility. In order to exert such effects, it is preferable to contain them by 0.0001%
or more in total. Although the content is preferable to be as much as possible when
only the properties are considered, because the effects saturate, 0.01% or less in
total is preferable. More preferable lower limit of these elements in total is 0.0002%
or more (further more preferably 0.0005% or more), and more preferable upper limit
in total is 0.005% or less (further more preferably 0.003% or less).
[0038] In the steel sheet for hot pressing use of the present invention, (A) some of Ti-containing
precipitates contained in the steel sheet, each of which having an equivalent circle
diameter of 30 nm or less, have an average equivalent circle diameter of 3 nm or less,
(B) relationship of precipitated Ti amount (mass%)-3.4[N]>0.5×[(total Ti amount (mass%))-3.4[N]]
(the relationship of the formula (1) described above) is fulfilled, and (C) the metal
microstructure contains at least either one of bainite and martensite, and the sum
total of the fraction of bainite and the fraction of martensite is 80 area% or more,
are also important requirements.
[0039] When Ti that is excessive with respect to N is dispersed finely or majority thereof
is present in a solid solution state in the steel sheet before hot press forming,
much amount of Ti comes to be present while it is fine in heating of hot press forming.
Thus, in martensite transformation that occurs during rapid cooling within the tool
after heating, the growth of martensite lath in the longitudinal direction is impeded,
the growth in the width direction is promoted, and the aspect ratio reduces. As a
result, discharge of carbon from the martensite lath to surrounding retained austenite
delays, the carbon amount in retained austenite reduces, the stability of retained
austenite deteriorates, and therefore the improvement effect of the elongation cannot
be obtained sufficiently.
[0040] From such a viewpoint, Ti-containing precipitates should be dispersed coarsely, and,
for that purpose, it is necessary that some of Ti-containing precipitates contained
in the steel sheet, each of which having an equivalent circle diameter of 30 nm or
less, have an average equivalent circle diameter of 3 nm or more (the requirement
of (A) described above). Also, the reason the equivalent circle diameter of the Ti-containing
precipitates of the object is stipulated to be 30 nm or less is that it is necessary
to control the Ti-containing precipitates and excluding TiN formed coarsely in the
melting stage that does not affect microstructure change and properties thereafter.
The size of the Ti-containing precipitates (the average equivalent circle diameter
of the Ti-containing precipitates whose equivalent circle diameter is 30 nm or less)
is preferably 5 nm or more, more preferably 10 nm or more. Further, the Ti-containing
precipitates of the object of the present invention also include precipitates containing
Ti such as TiVC, TiNbC, TiVCN, TiNbCN and the like in addition to TiC and TiN.
[0041] Also, in the steel sheet for hot pressing use, it is necessary that, out of Ti, majority
of Ti other than that used for precipitating and fixing N is present in the precipitated
state. For that purpose, it is necessary that the Ti amount present as the precipitates
other than TiN (that is, precipitated Ti amount (mass%)-3.4[N]) is more than 0.5 times
of the balance obtained by deducting Ti that forms TiN from total Ti (that is, more
than 0.5×[total Ti amount (mass%)-3.4[N]]) (the requirement of (B) described above).
Precipitated Ti amount (mass%)-3.4[N] is preferably 0.6×[total Ti amount (mass%)-3.4[N]]
or more, more preferably 0.7×[total Ti amount (mass%)-3.4[N]] or more.
[0042] Although control of the metal microstructure is intrinsically necessary for achieving
desired strength-elongation balance in the formed product, the metal microstructure
cannot be controlled only by the hot pressing condition, and it is necessary to control
the microstructure of the raw material steel thereof (the steel sheet for hot pressing
use) beforehand. In order to secure the proper amount of annealed martensite and annealed
bainite which are fine and largely contributing to ductility in the press forming
steel sheet, it is necessary to make the sum total of the fraction of bainite and
the fraction of martensite in the steel sheet 80 area% or more. When the sum total
of the fraction of bainite and the fraction of martensite is less than 80 area%, the
fraction of annealed martensite and/or annealed bainite targeted is hardly secured,
and the amount of other microstructure (ferrite for example) increases to deteriorate
the strength-elongation balance. The sum total of the fraction of bainite and the
fraction of martensite is preferably 90 area% or more, more preferably 95 area% or
more.
[0043] Further, in the steel sheet for hot pressing use of the present invention, although
the remainder of the metal microstructure is not particularly limited, at least any
of ferrite, pearlite or retained austenite can be cited for example.
[0044] The steel sheet (the steel sheet for hot pressing use) of the present invention as
described above can be manufactured by that a billet obtained by melting steel having
the chemical component composition as described above is subjected to hot rolling
with the heating temperature: 1,100°C or above (preferably 1,150°C or above) and 1,300°C
or below (preferably 1,250°C or below) and the finish rolling temperature of 750°C
or above (preferably 780°C or above) and 850°C or below (preferably 830°C or below),
cooling thereafter (slow cooling: intermediate cooling) so as to stay for 10 s or
more (preferably 50 s or more) between 700-750°C (preferably 720-740°C), cooling (rapid
cooling) thereafter to 450°C or below (preferably 350°C or below) at 20°C/s or more
(preferably 30°C/s or more), and winding at 100°C or above (preferably 150°C or above)
and 450°C or below (preferably 400°C or below).
[0045] The method described above is for executing control so that (1) rolling is finished
at a temperature range where dislocation introduced by hot rolling remains within
austenite, (2) Ti-containing precipitates such as TiC and the like are formed finely
on the dislocation by rapid cooling immediately thereafter, and (3) bainite transformation
or martensite transformation is caused by rapid cooling and winding thereafter.
[0046] The steel sheet for hot pressing use having the chemical component composition, metal
microstructure and Ti-precipitation state as described above may be used as it is
for manufacturing by hot press forming, and may be subjected to cold rolling with
the draft: 10-80% (preferably 20-70%) after pickling. Further, the steel sheet for
hot pressing use or the material obtained by cold rolling thereof may be subjected
to such heat treatment of heating to such a temperature range where TiC is not dissolved
by 100% (1,000°C or below: for example 870-900°C), rapidly cooling thereafter to 450°C
or below (preferably 400°C or below) at a cooling rate of 20°C/s or more (preferably
30°C/s or more), and holding thereafter at 450°C or below for 10 s or more and 1,000
s or less or tempering at a temperature of 450°C or below. Also, the steel sheet for
hot pressing use of the present invention may be subjected to plating containing at
least one element out of Al, Zn, Mg and Si on the surface thereof (the surface of
the base steel sheet).
[0047] By using the steel sheet for hot pressing use as described above, executing heating
to a temperature of Ac
1 transformation point+20°C or above and Ac
3 transformation point-20°C or below, thereafter starting press-forming, and executing
cooling to a temperature or below, the temperature being lower than the bainite transformation
starting temperature Bs by 100°C, while securing the average cooling rate of 20°C/s
or more within the tool during forming and after completion of forming, the press
formed product having a single property (may be hereinafter referred to as "single
region formed product") can have an optimum microstructure of low strength and high
ductility. The reasons for stipulating each requirement in this forming method are
as described below.
[0048] In order to form austenite between laths of martensite and bainite within the steel
sheet and to form annealed martensite and annealed bainite excellent in ductility
by annealing martensite and bainite, the heating temperature should be controlled
to a predetermined range. When the heating temperature of the steel sheet is below
Ac
1 transformation point+20°C, sufficient amount of austenite cannot be secured in heating,
and a predetermined amount of retained austenite cannot be secured in the final microstructure
(the microstructure of the formed product). Also, when the heating temperature of
the steel sheet exceeds Ac
3 transformation point-20°C, the transformation amount to austenite increases excessively
in heating, and a predetermined amount of annealed martensite and annealed bainite
cannot be secured in the final microstructure (the microstructure of the formed product).
[0049] In order to make austenite formed in the heating step described above a desired microstructure
while preventing formation of the microstructure such as ferrite or pearlite, it is
necessary to properly control the average cooling rate and the cooling finishing temperature
during forming and after forming. From such a viewpoint, it is necessary to make the
average cooling rate during forming 20°C/s or more and to make the cooling finishing
temperature a temperature or below, the temperature being lower than the bainite transformation
starting temperature Bs by 100°C. The average cooling rate during forming is preferably
30°C/s or more (more preferably 40°C/s or more). By transforming austenite having
been present in heating to bainite and martensite while preventing formation of the
microstructure such as ferrite or martensite by making the cooling finishing temperature
a temperature equal to or below the bainite transformation starting temperature Bs,
fine austenite is made remain between the laths of bainite and martensite, and a predetermined
amount of retained austenite is secured while securing bainite and martensite.
[0050] When the cooling finishing temperature becomes higher than the temperature that is
lower than the bainite transformation starting temperature Bs by 100°C and the average
cooling rate is less than 20°C/s, the microstructure such as ferrite, pearlite and
the like is formed, a predetermined amount of retained austenite cannot be secured,
and elongation (ductility) in the formed product deteriorates.
[0051] Although control of the average cooling rate basically becomes unnecessary at the
stage the temperature becomes equal to or below the temperature lower than the bainite
transformation starting temperature Bs by 100°C, cooling may be executed to the room
temperature with the average cooling rate of 1°C/s or more and 100°C/s or less for
example. Also, control of the average cooling rate during forming and after completion
of forming can be achieved by means such as (a) to control the temperature of the
forming tool (the cooling medium shown in Fig. 1 above), and (b) to control the thermal
conductivity of the tool.
[0052] In the press-formed product manufactured by hot press forming as described above,
the metal microstructure is formed of retained austenite: 3-20 area%, annealed martensite
and/or annealed bainite: 30-87 area%, and martensite as quenched: 10-67 area%, the
carbon amount in the retained austenite is 0.60% or more, and the balance of high
strength and elongation can be achieved with a high level and as a uniform property
within the formed product. The reasons for setting the range of each requirement (the
basic microstructure and the carbon amount in the retained austenite) in such a hot
press-formed product are as described below.
[0053] Retained austenite has an effect of increasing the work hardening ratio (transformation
induced plasticity) and improving ductility of the press-formed product by being transformed
to martensite during plastic deformation. In order to exert such an effect, the fraction
of retained austenite should be made 3 area% or more. Ductility becomes more excellent
as the fraction of retained austenite is higher. In the composition used for a steel
sheet for an automobile, retained austenite that can be secured is limited, and approximately
20 area% becomes the upper limit. Preferable lower limit of retained austenite is
5 area% or more (more preferably 7 area% or more).
[0054] By making the main microstructure annealed martensite and/or annealed bainite which
is fine and has low dislocation density, ductility (elongation) of the press-formed
product can be enhanced while securing a predetermined strength. From such a viewpoint,
the fraction of annealed martensite and/or annealed bainite is made 30 area% or more.
However, when this fraction exceeds 87 area%, the fraction of retained austenite becomes
insufficient, and ductility (residual ductility) deteriorates. Preferable lower limit
of annealed martensite and/or annealed bainite is 40 area% or more (more preferably
50 area% or more), and preferable upper limit is less than 80 area% (more preferably
less than 70 area%).
[0055] Because martensite as quenched is a microstructure inferior in ductility, when much
amount thereof is present, elongation is deteriorated, however, in order to achieve
high strength of over 100 kg/mm
2 class in a microstructure with low matrix strength such as annealed martensite, it
is necessary to secure a predetermined amount of martensite as quenched. From such
a viewpoint, the fraction of martensite as quenched is made 10 area% or more. However,
when the fraction of martensite as quenched increases excessively, strength increases
excessively and elongation becomes insufficient, and therefore the fraction thereof
should be 67 area% or less. Preferable lower limit of the fraction of martensite as
quenched is 20 area% or more (more preferably 30 area% or more), and preferable upper
limit is 60 area% or less (more preferably 50 area% or less).
[0056] With respect to the microstructure other the above, ferrite, pearlite, bainite and
the like may be included as the remainder microstructure, however, these microstructures
are inferior in contribution to strength and contribution to ductility compared to
other microstructures, and it is basically preferable not to be contained (it may
also be 0 area%). However, up to 20 area% is allowable. The remainder microstructure
is preferably 10 area% or less, more preferably 5 area% or less.
[0057] The carbon amount in retained austenite affects the timing of work induced transformation
of retained austenite to martensite at the time of deformation such as the tensile
test and the like, and enhances the transformation induced plasticity (TRIP) effect
by causing the work induced transformation at a higher strain zone as the carbon amount
is higher. In the case of the process of the present invention, carbon is discharged
during cooling from the martensite lath formed to surrounding austenite. At that time,
if Ti-carbide or carbonitride dispersed in steel is dispersed coarsely, growth of
the martensite lath in the longitudinal direction proceeds without being impeded,
and therefore the martensite lath narrow in the width, long, and having a large aspect
ratio is obtained. As a result, carbon is easily discharged from the martensite lath
to the width direction, the carbon amount in retained austenite increases, and the
ductility improves. From such a viewpoint, in the press-formed product of the present
invention, the carbon amount in retained austenite in steel was stipulated to be 0.60%
or more. Further, although the carbon amount in retained austenite can be concentrated
to approximately 0.70%, approximately 1.0% is the limit.
[0058] When the steel sheet for hot pressing use of the present invention is used, by properly
adjusting the press forming condition (heating temperature and cooling rate), the
properties such as strength, elongation and the like of the press-formed product can
be controlled, the press-formed product with high ductility (residual ductility) is
obtained, and therefore application to a portion (energy absorption member for example)
to which it has been difficult to apply conventional press-formed products becomes
also possible which is very useful in expanding the application range of the press-formed
product. Also, not only the single region formed product described above, a press-formed
product exerting strength-ductility balance according to each region (may be hereinafter
referred to as "plural region formed product") is obtained when the heating temperature
and the condition of each region in forming are properly controlled and the microstructure
of each region is adjusted in manufacturing the press-formed product by press forming
of a steel sheet using a press-forming tool.
[0059] The plural region formed product can be manufactured as described above using the
steel sheet for hot pressing use of the present invention by dividing a heating region
of the steel sheet into at least two regions, heating one region thereof (hereinafter
referred to as the first region) to a temperature of Ac
3 transformation point or above and 950°C or below, heating another region (hereinafter
referred to as the second region) to a temperature of Ac
1 transformation point+20°C or above and Ac
3 transformation point-20°C or below, thereafter starting press forming of both of
the first and second regions, and executing cooling to a temperature of martensite
transformation starting temperature Ms or below while securing the average cooling
rate of 20°C/s or more within a tool in both of the first and second regions during
forming and after forming.
[0060] According to the method described above, by dividing the heating region of the steel
sheet into at least two regions (high strength side region and low strength side region)
and controlling the manufacturing condition according to each region, such a press-formed
product that strength-ductility balance according to each region is exerted is obtained.
The second region out of two regions corresponds to the low strength side region,
and the manufacturing condition, microstructure and properties in this region is basically
same to those of the single region formed product described above. Below, the manufacturing
condition for forming the other first region (corresponding to the high strength side
region) will be described. Also, in executing this manufacturing method, it is required
to form regions with different heating temperature by a single steel sheet, however,
by using an existing heating furnace (for example, far infrared furnace, electric
furnace+shield), controlling while making the boundary section of the temperature
50 mm or less is possible.
(Manufacturing condition of the first region/high strength side region)
[0061] In order to properly adjust the microstructure of the press-formed product, it is
necessary to control the heating temperature to a predetermined range. By properly
controlling this heating temperature, transformation to a microstructure mainly of
martensite is caused while securing a predetermined amount of retained austenite in
the cooling step after heating, and a desired microstructure can be achieved within
the range of the final hot press-formed product. When the steel sheet heating temperature
in this region is below Ac
3 transformation point, a sufficient amount of austenite cannot be obtained in heating,
and a predetermined amount of retained austenite cannot be secured in the final microstructure
(the microstructure of the formed product). Also, when the heating temperature of
the steel sheet exceeds 950°C, the grain size of austenite becomes large in heating,
martensite transformation starting temperature (Ms point) and martensite transformation
finishing temperature (Mf point) rise, retained austenite cannot be secured in quenching,
and excellent formability is not achieved. The heating temperature of the steel sheet
is preferably Ac
3 transformation point+50°C or above and 900°C or below.
[0062] In order to make austenite formed in the heating step described above a desired microstructure
while preventing formation of the microstructure such as ferrite or pearlite, it is
necessary to properly control the average cooling rate and the cooling finishing temperature
during forming and after forming. From such a viewpoint, the average cooling rate
during forming should be 20°C/s or more and the cooling finishing temperature should
be martensite transformation starting temperature (Ms point) or below. The average
cooling rate during forming is preferably 30°C/s or more (more preferably 40°C/s or
more). By transforming austenite having been present in heating to martensite while
preventing formation of the microstructure such as ferrite or pearlite by making the
cooling finishing temperature the martensite transformation starting temperature (Ms
point) or below, martensite is secured. Specifically, the cooling finishing temperature
is 400°C or below, preferably 300°C or below.
[0063] In the press-formed product obtained by such a method, the metal microstructure,
precipitates and the like are different between the first region and the second region.
In the first region, the metal microstructure is of retained austenite: 3-20 area%
(the action and effect of retained austenite are same to the above), and martensite:
80 area% or more. In the second region, the metal microstructure same to that of the
single region formed product described above and 0.60% or more of the carbon amount
in retained austenite are fulfilled.
[0064] By making the main microstructure of the first region martensite with high strength
containing a predetermined amount of retained austenite, ductility and high strength
in a specific region in the hot press-formed product can be secured. From such a viewpoint,
the area fraction of martensite should be 80 area% or more. The fraction of martensite
is preferably 85 area% or more (more preferably 90 area% or more). Also, as the microstructure
in the first region, ferrite, pearlite, bainite and the like may be included in a
part thereof.
[0065] Although the effect of the present invention will be shown below more specifically
by examples, the examples described below do not limit the present invention, and
any of the design alterations judging from the purposes described above and below
is to be included in the technical range of the present invention.
[Examples]
[Example 1]
[0066] Steel (steel Nos. 1-32) having the chemical component composition shown in Table
1 below was molten in vacuum, was made a slab for experiment, was thereafter made
a steel sheet by hot rolling, was thereafter cooled, and was subjected to a treatment
that simulates winding (sheet thickness: 3.0 mm). The winding simulated treatment
method included cooling to the winding temperature, putting the sample thereafter
into a furnace heated to the winding temperature, holding for 30 min, and cooling
in the furnace. The manufacturing condition for the steel sheet at that time is shown
in Table 2 below. Also, Ac
1 transformation point, Ac
3 transformation point, Ms point, and Bs point in Table 1 were obtained using the formula
(2)-formula (5) below (refer to "The physical Metallurgy of Steels", Leslie, Maruzen
Company, Limited (1985) for example). Also, the treatments (1)-(3) shown in the remarks
column in Table 2 express that each treatment (rolling, cooling, alloying) shown below
was executed.
wherein [C], [Si], [Mn], [P], [Al], [Ti], [V], [Cr], [Mo], [Cu] and [Ni] represent
the content (mass%) of C, Si, Mn, P, Al, Ti, V, Cr, Mo, Cu and Ni respectively. Also,
when the element shown in each term of the formulae (2)-(5) above is not contained,
calculation is done assuming that the term is null.
[0067] Treatment (1): After finish rolling, cooling was executed to 650°C with the average
cooling rate of 50°C/s, cooling was thereafter executed for 10 s from 650°C with the
average cooling rate of 5°C/s, and cooling was thereafter executed to the winding
temperature with the average cooling rate of 50°C/s. The front and back surfaces were
thereafter polished and the thickness was reduced to 1.6 mm so as to match the thickness
to that of the treatments (2) and (3).
[0068] Treatment (2): The hot-rolled steel sheet was cold-rolled, was heated thereafter
to 860°C simulating continuous annealing, was cooled thereafter to 400°C with the
average cooling rate of 30°C/s, and was held.
[0069] Treatment (3): The hot-rolled steel sheet was cold-rolled, was heated thereafter
to 860°C for simulating continuous hot dip galvanizing line, was cooled thereafter
to 400°C with the average cooling rate of 30°C/s, was held, was thereafter heated
further by (500°C×10 s), and was cooled thereafter.
[0070]
[Table 2]
Steel No. |
Steel sheet manufacturing condition |
Heating temperature (°C) |
Finish rolling temperature (°C) |
Cooling time of 750-700°C (s) |
Average cooling rate of 700°C-450°C (°C/s) |
Winding temperature (°C) |
Remarks |
1 |
1200 |
800 |
15 |
50 |
200 |
- |
2 |
1200 |
800 |
15 |
50 |
200 |
- |
3 |
1200 |
800 |
15 |
50 |
200 |
- |
4 |
1200 |
800 |
15 |
50 |
200 |
- |
5 |
1200 |
800 |
15 |
50 |
200 |
- |
6 |
1200 |
800 |
15 |
50 |
200 |
- |
7 |
1200 |
800 |
15 |
50 |
200 |
- |
8 |
1200 |
800 |
15 |
50 |
200 |
- |
9 |
1200 |
800 |
1 |
17 |
200 |
Treatment (1) |
10 |
1200 |
900 |
1 |
17 |
450 |
Treatment (1) |
11 |
1200 |
800 |
15 |
50 |
400 |
Cold rolling: 20% |
12 |
1200 |
800 |
15 |
50 |
400 |
Treatment (2) |
13 |
1200 |
800 |
15 |
50 |
400 |
Treatment (3) |
14 |
1200 |
800 |
15 |
50 |
580 |
- |
15 |
1200 |
800 |
15 |
50 |
200 |
- |
16 |
1200 |
800 |
15 |
50 |
200 |
- |
17 |
1200 |
800 |
15 |
50 |
200 |
- |
18 |
1200 |
800 |
15 |
50 |
200 |
- |
19 |
1200 |
800 |
15 |
50 |
200 |
- |
20 |
1200 |
800 |
15 |
50 |
200 |
- |
21 |
1200 |
800 |
15 |
50 |
200 |
- |
22 |
1200 |
800 |
15 |
50 |
200 |
- |
23 |
1200 |
800 |
15 |
50 |
200 |
- |
24 |
1200 |
800 |
15 |
50 |
200 |
- |
25 |
1200 |
800 |
15 |
50 |
200 |
- |
26 |
1200 |
800 |
15 |
50 |
200 |
- |
27 |
1200 |
800 |
15 |
50 |
200 |
- |
28 |
1200 |
800 |
15 |
50 |
200 |
- |
[Analysis of precipitation state of Ti of steel sheet]
[0071] An extraction replica sample was prepared, and a transmission electron microscope
image (magnifications: 100,000 times) of Ti-containing precipitates was photographed
using a transmission electron microscope (TEM). At this time, by composition analysis
of the precipitates using an energy dispersion type X-ray spectrometer (EDX), Ti-containing
precipitates were identified. The area of the Ti-containing precipitates of at least
100 pieces was measured by image analysis, those having the equivalent circle diameter
of 30 nm or less were extracted, and the average value thereof was made the size of
the precipitates. Also, in the table, the size is shown as "average equivalent circle
diameter of Ti-containing precipitates". Further, with respect to precipitated Ti
amount (mass%)-3.4[N] (the Ti amount present as the precipitates), extraction residue
analysis (in extraction treatment, the precipitates coagulate, and fine precipitates
also can be measured) was executed using a mesh with mesh diameter: 0.1 µm, and precipitated
Ti amount (mass%)-3.4[N] (expressed as "precipitated Ti amount-3.4[N]" in Table 3)
was obtained. Also, when the Ti-containing precipitates partly contained V and Nb,
the contents of these precipitates were also measured.
[Observation of metal microstructure (fraction of each microstructure)]
[0072]
- (1) With respect to the microstructure of martensite and bainite in the steel sheet,
the steel sheet was corroded by nital, martensite and bainite were distinguished from
each other by SEM observation (magnifications: 1,000 times or 2,000 times), and each
fraction (area ratio) was obtained.
- (2) The retained austenite fraction in the steel sheet was measured by X-ray diffraction
method after the steel sheet was ground up to 1/4 thickness thereof and was thereafter
subjected to chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7, P.776).
[0073]
[Table 3]
Steel No. |
Steel sheet for press forming use |
Precipitated Ti amount - 3.4[N] (mass%) |
0.5 x (total Ti amount - 3.4[N]) (mass%) |
Average equivalent circle diameter of Ti-containing precipitates (nm) |
Fraction of martensite (area%) |
Fraction of bainite (area%) |
Others |
1 |
0.024 |
0.015 |
10.0 |
100 |
0 |
- |
2 |
0.024 |
0.015 |
10.0 |
100 |
0 |
- |
3 |
0.025 |
0.015 |
10.0 |
100 |
0 |
- |
4 |
0.027 |
0.015 |
10.0 |
100 |
0 |
- |
5 |
0.029 |
0.015 |
10.0 |
100 |
0 |
- |
6 |
0.026 |
0.015 |
10.0 |
100 |
0 |
- |
7 |
0.023 |
0.015 |
10.0 |
100 |
0 |
- |
8 |
0.026 |
0.015 |
10.0 |
100 |
0 |
- |
9 |
0.006 |
0.015 |
5.0 |
100 |
0 |
- |
10 |
0.012 |
0.015 |
2.0 |
0 |
100 |
- |
11 |
0.026 |
0.015 |
10.0 |
0 |
100 |
- |
12 |
0.030 |
0.015 |
10.0 |
0 |
100 |
- |
13 |
0.027 |
0.015 |
10.0 |
0 |
100 |
- |
14 |
0.025 |
0.015 |
10.0 |
0 |
0 |
Ferrite + Pearlite: 100% |
15 |
0.028 |
0.015 |
10.0 |
100 |
0 |
- |
16 |
0.023 |
0.015 |
10.0 |
90 |
0 |
Retained austenite 10% |
17 |
0.026 |
0.015 |
10.0 |
80 |
0 |
Retained austenite 20% |
18 |
0.030 |
0.015 |
10.0 |
60 |
0 |
Retained austenite 40% |
19 |
0.023 |
0.015 |
10.0 |
100 |
0 |
- |
20 |
0.023 |
0.015 |
10.0 |
100 |
0 |
- |
21 |
0.084 |
0.043 |
10.0 |
100 |
0 |
- |
22 |
0.144 |
0.093 |
18.0 |
100 |
0 |
- |
23 |
0.029 |
0.015 |
10.0 |
100 |
0 |
- |
24 |
0.024 |
0.015 |
10.0 |
100 |
0 |
- |
25 |
0.024 |
0.015 |
10.0 |
100 |
0 |
- |
26 |
0.026 |
0.015 |
10.0 |
100 |
0 |
- |
27 |
0.028 |
0.015 |
10.0 |
100 |
0 |
- |
28 |
0.024 |
0.015 |
10.0 |
100 |
0 |
- |
[0074] Each steel sheet described above (1.6 mm
t×150 mm×200 mm) (with respect to those other than the treatments of (1)-(3) described
above, the thickness was adjusted to 1.6 mm by hot rolling) was heated to a predetermined
temperature in a heating furnace, and was thereafter subjected to press forming and
cooling treatment using the tool (Fig. 1 above) of a hat shape to obtain the press-formed
product. The press forming conditions (heating temperature, average cooling rate,
and rapid cooling finishing temperature in press forming) are shown in Table 4 below.
[0075]
[Table 4]
Steel No. |
Press forming condition |
Heating temperature (°C) |
Average cooling rate (°C/s) |
Rapid cooling finishing temperature (°C) |
1 |
790 |
40 |
300 |
2 |
800 |
40 |
300 |
3 |
760 |
40 |
300 |
4 |
770 |
40 |
300 |
5 |
810 |
40 |
300 |
6 |
950 |
40 |
300 |
7 |
820 |
5 |
300 |
8 |
800 |
40 |
600 |
9 |
800 |
40 |
300 |
10 |
810 |
40 |
300 |
11 |
800 |
40 |
300 |
12 |
820 |
40 |
300 |
13 |
810 |
40 |
300 |
14 |
800 |
40 |
300 |
15 |
830 |
40 |
300 |
16 |
790 |
40 |
300 |
17 |
790 |
40 |
300 |
18 |
770 |
40 |
300 |
19 |
800 |
40 |
300 |
20 |
780 |
40 |
300 |
21 |
810 |
40 |
300 |
22 |
820 |
40 |
300 |
23 |
830 |
40 |
300 |
24 |
800 |
40 |
300 |
25 |
810 |
40 |
300 |
26 |
800 |
40 |
300 |
27 |
810 |
40 |
300 |
28 |
820 |
40 |
300 |
[0076] With respect to the formed product obtained, tensile strength (TS), elongation (total
elongation EL), and observation of the metal microstructure (the fraction of each
microstructure) were measured by methods described below.
[Measurement of tensile strength (TS) and elongation (total elongation EL)]
[0077] The tensile test was executed using JIS No. 5 test specimen, and the tensile strength
(TS) and the elongation (EL) were measured. At this time, the strain rate of the tensile
test was made 10 mm/s. In the present invention, the case 980-1,179 MPa of the tensile
strength (TS) and 20% or more of the elongation (EL) were satisfied and the strength-elongation
balance (TS×EL) was 24,000 (MPa ·%) or more was evaluated to have passed.
(Observation of metal microstructure (fraction of each microstructure))
[0078]
- (1) With respect to the microstructure of annealed martensite, bainite and annealed
bainite in the steel sheet, the steel sheet was corroded by nital, annealed martensite,
bainite and annealed bainite were distinguished from each other by SEM observation
(magnifications: 1,000 times or 2,000 times), and each fraction (area ratio) was obtained.
- (2) The retained austenite fraction in the steel sheet was measured by X-ray diffraction
method after the steel sheet was ground up to 1/4 thickness thereof and was thereafter
subjected to chemical polishing (for example, ISJJ Int. Vol. 33. (1933), No. 7, P.776). At this time, the carbon amount in retained austenite was also measured.
- (3) With respect to the fraction of martensite as quenched, the steel sheet was LePera-corroded,
the area ratio of the white contrast was measured as the mixture microstructure of
martensite as quenched and retained austenite, the retained austenite fraction obtained
by X-ray diffraction was deducted therefrom, and the fraction of martensite as quenched
was calculated.
[0079] The observation results (fraction of each microstructure) of the metal microstructure
are shown in Table 5 below. Also, the mechanical properties (tensile strength TS,
elongation EL, and TS×EL) of the formed product are shown in Table 6 below.
[0080]
[Table 5]
Steel No. |
Metal microstructure of formed product |
Fraction of annealed martensite and/or annealed bainite (area%) |
Fraction of martensite as quenched (area%) |
Fraction of retained austenite (area%) |
Carbon amount in retained austenite (mass%) |
Others |
1 |
70 |
23 |
7 |
0.65 |
- |
2 |
67 |
23 |
7 |
0.65 |
Ferrite 3% |
3 |
70 |
28 |
2 |
0.45 |
- |
4 |
70 |
25 |
5 |
0.65 |
- |
5 |
70 |
23 |
7 |
0.65 |
- |
6 |
70 |
23 |
7 |
0.65 |
- |
7 |
70 |
2 |
0 |
0.65 |
Pearlite 18%, Ferrite 10% |
8 |
70 |
6 |
3 |
0.65 |
Pearlite 12%, Ferrite 9% |
9 |
70 |
23 |
7 |
0.65 |
- |
10 |
70 |
23 |
7 |
0.65 |
- |
11 |
70 |
23 |
7 |
0.65 |
- |
12 |
70 |
23 |
7 |
0.65 |
- |
13 |
70 |
23 |
7 |
0.65 |
- |
14 |
- |
33 |
7 |
0.52 |
Ferrite 60% |
15 |
70 |
23 |
9 |
0.65 |
- |
16 |
60 |
29 |
11 |
0.65 |
- |
17 |
55 |
32 |
13 |
0.65 |
- |
18 |
16 |
62 |
22 |
0.65 |
- |
19 |
70 |
25 |
5 |
0.68 |
- |
20 |
70 |
21 |
9 |
0.61 |
- |
21 |
70 |
23 |
7 |
0.65 |
- |
22 |
70 |
23 |
7 |
0.65 |
- |
23 |
70 |
23 |
7 |
0.65 |
- |
24 |
70 |
23 |
7 |
0.65 |
- |
25 |
70 |
23 |
7 |
0.65 |
- |
26 |
70 |
23 |
7 |
0.65 |
- |
27 |
70 |
23 |
7 |
0.65 |
- |
28 |
70 |
23 |
7 |
0.65 |
- |
[0081]
[Table 6]
Steel No. |
Mechanical properties of formed product |
Tensile strength TS (MPa) |
Elongation EL (%) |
TS×EL (MPa·%) |
1 |
1074 |
23.5 |
25239 |
2 |
1022 |
26.1 |
26674 |
3 |
1014 |
12.0 |
26364 |
4 |
983 |
25.5 |
25067 |
5 |
1016 |
26.2 |
26619 |
6 |
1524 |
11.3 |
17221 |
7 |
781 |
19.7 |
15386 |
8 |
856 |
19.0 |
16264 |
9 |
1026 |
22.6 |
23188 |
10 |
1044 |
22.0 |
22968 |
11 |
1051 |
25.9 |
27221 |
12 |
1018 |
25.3 |
25755 |
13 |
1039 |
24.1 |
25040 |
14 |
1063 |
19.9 |
21154 |
15 |
1038 |
25.6 |
26573 |
16 |
1192 |
23.5 |
28012 |
17 |
1260 |
28.0 |
35280 |
18 |
1321 |
10.2 |
13474 |
19 |
1031 |
24.3 |
25053 |
20 |
1021 |
29.0 |
29609 |
21 |
1045 |
25.9 |
27066 |
22 |
1045 |
18.1 |
18915 |
23 |
1013 |
26.1 |
26439 |
24 |
1049 |
24.0 |
25176 |
25 |
1033 |
26.0 |
26858 |
26 |
1007 |
26.1 |
26283 |
27 |
1059 |
25.2 |
26687 |
28 |
1045 |
24.2 |
25289 |
[0082] From these results, following consideration can be made. Those of the steel Nos.
1, 2, 4, 5, 11-13, 15-17, 19-21, 23-32 are examples fulfilling the requirements stipulated
in the present invention, and it is known that components excellent in strength-elongation
balance have been obtained.
[0083] On the other hand, those of the steel Nos. 3, 6-10, 14, 18, 22 are the comparative
examples not fulfilling any of the requirements stipulated in the present invention,
and any of the properties is deteriorated. That is, in that of the steel No. 3, a
steel sheet with low Si content is used, the fraction of retained austenite in the
formed product is not secured, the carbon amount in the retained austenite drops,
and the elongation is not enough. In that of the steel No. 6, the heating temperature
in forming is high, only low elongation EL is obtained, and the strength-elongation
balance (TS×EL) also deteriorates.
[0084] In that of the steel No. 7, the average cooling rate in press forming is slow, pearlite
and ferrite are formed, the fraction of martensite as quenched cannot be secured,
and the strength-elongation balance (TS×EL) is deteriorated. In that of the steel
No. 8, the rapid cooling finishing temperature is high, pearlite and ferrite are formed,
the fraction of martensite as quenched cannot be secured, only low elongation is obtained,
and the strength-elongation balance (TS×EL) is also deteriorated.
[0085] In those of the steel Nos. 9, 10, the condition in manufacturing the steel is not
appropriate, the amount of precipitated Ti is insufficient (steel Nos. 9, 10), Ti-containing
precipitates are small (steel No. 10), and, when press forming is executed using such
a steel sheet, the strength-elongation balance (TS×EL) is deteriorated even if the
forming condition is appropriate.
[0086] In that of the steel No. 14, a steel sheet whose metal microstructure is of ferrite+pearlite
of 100 area% which is caused by the winding temperature is used, the fraction of annealed
martensite and/or annealed bainite during forming cannot be secured, and the strength-elongation
balance (TS×EL) is deteriorated. In that of the steel No. 18, the steel sheet with
excessive C content is used, the strength becomes high, and only low elongation EL
is obtained. In that of the steel No. 22, the steel sheet with excessive Ti content
is used, and the strength-elongation balance (TS×EL) is deteriorated.
[Example 2]
[0087] Steel (steel Nos. 33-37) having the chemical component composition shown in Table
7 below was molten in vacuum, was made a slab for experiment, was thereafter hot-rolled,
and was thereafter cooled and wound (sheet thickness: 3.0 mm). The manufacturing condition
for the steel sheet at that time is shown in Table 8 below.
[0088]
[Table 7]
Steel No. |
Chemical component composition* (mass%) |
Ac3-20°C(°C) |
Ac1+20°C(°C) |
Bs-100°C(°C) |
Ms point (°C) |
C |
Si |
Mn |
P |
S |
Al |
B |
Ti |
N |
V |
Nb |
Cu |
Ni |
Cr |
Mo |
29 |
0.220 |
1.20 |
1.20 |
0.0050 |
0.0020 |
0.030 |
0.0020 |
0.044 |
0.0040 |
- |
- |
- |
- |
0.20 |
- |
843 |
768 |
549 |
421 |
30 |
0.220 |
1.20 |
1.20 |
0.0050 |
0.0020 |
0.030 |
0.0020 |
0.044 |
0.0040 |
- |
- |
- |
- |
0.20 |
- |
843 |
768 |
549 |
421 |
31 |
0.350 |
1.20 |
1.20 |
0.0050 |
0.0020 |
0.030 |
0.0020 |
0.044 |
0.0040 |
- |
- |
- |
- |
0.20 |
- |
818 |
768 |
514 |
374 |
32 |
0.220 |
1.20 |
1.20 |
0.0050 |
0.0020 |
0.030 |
0.0020 |
0.044 |
0.0040 |
- |
- |
- |
- |
0.20 |
- |
843 |
768 |
549 |
421 |
*The remainder: iron and inevitable impurities other than P, S, N. |
[0089]
[Table 8]
Steel No. |
Steel sheet manufacturing condition |
Heating temperature (°C) |
Finish rolling temperature (°C) |
Cooling time of 750-700°C (s) |
Average cooling rate of 700°C-450°C (°C/s) |
Winding temperature (°C) |
Remarks |
29 |
1200 |
800 |
15 |
50 |
200 |
- |
30 |
1200 |
800 |
15 |
50 |
200 |
- |
31 |
1200 |
800 |
15 |
50 |
200 |
- |
32 |
1200 |
800 |
1 |
17 |
200 |
Treatment (1) |
[0090] With respect to the steel sheet obtained, analysis of the precipitation state of
Ti-containing precipitates and observation of the metal microstructure (the fraction
of each microstructure) were executed similarly to Example 1. The result is shown
in Table 9 below.
[0091]
[Table 9]
Steel No. |
Steel sheet for press forming use |
Precipitated Ti amount - 3.4[N] (mass%) |
0.5 x (total Ti amount - 3.4[N]) (mass%) |
Average equivalent circle diameter of Ti-containing precipitates (nm) |
Fraction of martensite (area%) |
Fraction of bainite (area%) |
Others |
29 |
0.030 |
0.015 |
10.0 |
100 |
0 |
- |
30 |
0.023 |
0.015 |
10.0 |
100 |
0 |
- |
31 |
0.028 |
0.015 |
10.0 |
90 |
0 |
Retained austenite 10% |
32 |
0.006 |
0.015 |
2.3 |
100 |
0 |
- |
[0092] Each steel sheet described above (3.0 mm
t×150 mm×200 mm) was heated to a predetermined temperature in a heating furnace, and
was subjected thereafter to press forming and cooling treatment using the tool (Fig.
1 above) of a hat shape to obtain the formed product. At this time, the steel sheet
was put in an infrared furnace and the portion intended to be high-strengthened (the
steel sheet portion corresponding to the first region) was configured so that infrared
rays directly hit so as to allow high temperature heating, whereas the portion intended
to be low-strengthened (the steel sheet portion corresponding to the second region)
was shielded with a cover so that a part of the infrared rays was blocked so as to
allow low temperature heating, and thereby the heating temperature was differentiated.
Therefore, the formed product has the regions with different strength within a single
component. The press forming conditions (heating temperature, average cooling rate,
and rapid cooling finishing temperature of each region in press forming) are shown
in Table 10 below.
[0093]
[Table 10]
Steel No. |
Region |
Press forming condition |
Heating temperature (°C) |
Average cooling rate (°C/s) |
Rapid cooling finishing temperature (°C) |
29 |
Low strength side |
810 |
40 |
300 |
High strength side |
920 |
40 |
300 |
30 |
Low strength side |
800 |
40 |
300 |
High strength side |
850 |
40 |
300 |
31 |
Low strength side |
790 |
40 |
300 |
High strength side |
920 |
40 |
300 |
32 |
Low strength side |
810 |
40 |
300 |
High strength side |
920 |
40 |
300 |
[0094] With respect to the formed product obtained, tensile strength (TS), elongation (total
elongation EL), observation of the metal microstructure (the fraction of each microstructure),
and the carbon amount in retained austenite in each region were obtained similarly
to Example 1.
[0095] The observation results (fraction of each microstructure) of the metal microstructure
are shown in Table 11 below. Also, the mechanical properties (tensile strength TS,
elongation EL, and TS×EL) of the formed product are shown in Table 12 below. Further,
the case 1,470 MPa or more of the tensile strength (TS) and 8% or more of the elongation
(EL) were fulfilled and the strength-elongation balance (TS×EL) was 14,000 (MPa ·%)
or more on the high strength side was evaluated to have passed (the evaluation criteria
of the low strength side are same to those of Example 1).
[0096]
[Table 11]
Steel No. |
Region |
Metal microstructure of formed product |
Fraction of annealed martensite and/or annealed bainite (area%) |
Fraction of martensite as quenched (area%) |
Fraction of retained austenite (area%) |
Carbon amount in retained austenite (mass%) |
Others |
29 |
Low strength side |
70 |
23 |
7 |
0.65 |
- |
High strength side |
0 |
94 |
6 |
0.55 |
- |
30 |
Low strength side |
70 |
23 |
7 |
0.65 |
- |
High strength side |
0 |
61 |
6 |
0.52 |
Ferrite 33% |
31 |
Low strength side |
60 |
29 |
11 |
0.65 |
- |
High strength side |
- |
95 |
5 |
0.53 |
- |
32 |
Low strength side |
70 |
23 |
7 |
0.65 |
- |
High strength side |
0 |
94 |
6 |
0.54 |
- |
[0097]
[Table 12]
Steel No. |
Region |
Mechanical properties of formed product |
Tensile strength TS (MPa) |
Elongation EL (%) |
TS × EL (MPa·%) |
29 |
Low strength side |
1058 |
24.1 |
25498 |
High strength side |
1511 |
11.0 |
16621 |
30 |
Low strength side |
1063 |
23.9 |
25406 |
High strength side |
1278 |
13.0 |
16614 |
31 |
Low strength side |
1192 |
25.0 |
29800 |
High strength side |
1820 |
10.1 |
18382 |
32 |
Low strength side |
1049 |
21.5 |
22554 |
High strength side |
1499 |
11.0 |
16489 |
[0098] From this result, following consideration can be made. Those of the steel Nos. 33,
35, 37 are examples fulfilling the requirements stipulated in the present invention,
and it is known that components excellent in the strength-elongation balance in each
region have been obtained.
[0099] On the other hand, those of the steel Nos. 34, 36 are the comparative examples not
fulfilling any of the requirements stipulated in the present invention, and any of
the properties is deteriorated. That is, in that of the steel No. 34, the heating
temperature in press forming is low, and the strength on the high strength side drops.
In that of the steel No. 36, a steel sheet with small size of Ti-containing precipitates
is used, only low strength is obtained on the high strength side, and the strength-elongation
balance (TS×EL) is deteriorated on the low strength side.
[0100] Although the present invention has been described in detail and referring to specific
embodiments, it is obvious for a person with an ordinary skill in the art that various
alterations and amendments can be effected without departing from the spirit and the
range of the present invention.
[0101] The present application is based on Japanese Patent Application (
JP-A-No. 2012-053844) applied on March 9, 2012, and the contents thereof are hereby incorporated by reference.
[Industrial Applicability]
[0102] The present invention is suitable to a steel sheet for hot pressing use that is used
in manufacturing structural components of an automobile.
[Reference Signs List]
[0103]
- 1...punch
- 2...die
- 3...blank holder
- 4...steel sheet (blank)